Nanocrystal structures

ABSTRACT

A structure including a grating and a semiconductor nanocrystal layer on the grating, can be a laser. The semiconductor nanocrystal layer can include a plurality of semiconductor nanocrystals including a Group II-VI compound, the nanocrystals being distributed in a metal oxide matrix. The grating can have a periodicity from 200 nm to 500 nm.

This application is a divisional of, and claims priority to, U.S. patentapplication Ser. No. 11/594,732, which is a divisional of U.S. patentapplication Ser. No. 10/294,742, filed Nov. 15, 2002, and claimspriority to U.S. patent application Ser. No. 60/331,454, filed on Nov.16, 2001, each of which is hereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant Nos.CHE9708265 and DMR9808941, awarded by the NSF and under Grant No.W-7405-ENG-36, awarded by DOE. The government has certain rights to thisinvention.

TECHNICAL FIELD

The present invention relates to structures and devices including asemiconductor nanocrystal.

BACKGROUND

Chemically synthesized colloidal semiconductor nanocrystals, alsoreferred to as quantum dots, consist of 1-10 nm diameter inorganicsemiconductor particles decorated with a layer of organic ligands. See,C. B. Murray et al., Annu. Rev. Mat. Sci., 2000, 30, 545-610, which isincorporated in its entirety. These zero-dimensional semiconductorstructures show strong quantum confinement effects that can be harnessedin designing bottom-up chemical approaches to create complexheterostructures with electronic and optical properties that are tunablewith the size of the nanocrystals. At the same time, as a result of thesurrounding organic ligand shell, semiconductor nanocrystals can bechemically manipulated as large molecules.

Optical amplifiers utilize a gain medium to amplify optical radiation.In an amplifier, a source excites the gain medium to produce apopulation inversion between high and low energy states of the gainmedium. The excited gain medium can amplify optical radiation atenergies overlapping the energy differences between the high and lowenergy states of the population inversion because stimulated emission ofradiation from the medium is more efficient than absorption of light. Ingeneral, a laser utilizes a cavity to supply feedback to an excited gainmedium to cause amplified spontaneous emission. A laser cavity caninclude a series of optical components, such as mirrors, arrangedrelative to the gain medium to reflect radiation back into the cavityand thereby provide feedback. For example, a gain medium can be placedinto a stable or unstable resonator. Conventional solid-state and gaslasers and amplifiers generally provide very specific spectral outputsdepending upon the laser material. If a spectral output other than thatachievable with available gain materials or a less specific spectraloutput is desired, dye lasers or tunable optical parametric oscillators(OPO) or amplifiers (OPA) can be used. Dye lasers are large and bulkyand also require fluid components that can be toxic.

SUMMARY

A structure includes a grating and a semiconductor nanocrystal layer.The semiconductor nanocrystal layer can include a plurality ofnanocrystals incorporated in an inorganic matrix. The inorganic matrixcan be a metal oxide matrix prepared, for example, by sol-gelprocessing, or other low temperature matrix-forming methods. The metaloxide matrix can be crystalline or non-crystalline. The metal oxidematrix can be free of light-scattering defects, such as, for example,cracks.

In one aspect, a structure includes grating having a periodicity from200 nm to 500 nm and a semiconductor nanocrystal layer disposed on thegrating.

In another aspect, a laser includes a grating and a semiconductornanocrystal layer disposed on the grating, the layer including aplurality of semiconductor nanocrystals including a Group II-VIcompound. The nanocrystals can be distributed in a metal oxide matrix.

In another aspect, a method of forming a laser includes selecting asemiconductor nanocrystal having a diameter and a composition andplacing the semiconductor nanocrystal on a grating. The diameter of thenanocrystal can be selected to produce a laser having a particularoutput energy, the composition of the nanocrystal can be selected toproduce a laser having a particular output energy, or the grating can beselected to have a periodicity selected to produce a laser having aparticular output energy, or combinations thereof. A layer including thesemiconductor nanocrystal can be formed on a surface of the grating.

Similar to previous efforts of incorporating nanocrystals into polymermatrices (see, J. W. Lee et al., Advanced Materials 2000, 12, 1102,incorporated by reference in its entirety), the synthesis of thenanocrystals and the preparation of the matrix are decoupled. Thus,narrow size distribution, high quality nanocrystals with highfluorescence efficiency are first prepared using previously establishedliterature procedures and used as the building blocks. See, C. B. Murrayet al., J. Amer. Chem. Soc, 1993, 115, 8706, B. O. Dabbousi et al., J.Phys. Chem. B 1997, 101, 9463, each of which is incorporated byreference in its entirety. The organic, surface-passivating ligands arethen exchanged to stabilize the nanocrystals in polar solvents likeethanol, and also to provide a tether with which the nanocrystals areincorporated into the titania sol-gel matrix. Formation of a titaniamatrix using a titanium (IV) alkoxide precursor exposed controllably tomoisture (see, A. Imhof et al., Nature 1997, 389, 948, incorporated byreference in its entirety) obviates the use of acid catalysts that aredetrimental to the optical properties of the nanocrystals. Thermalannealing then completes the composite preparation. In this process, thegelation time under an inert atmosphere is critical, as incompleteincorporation of the nanocrystals leads to microscale phase separationof the nanocrystals from the titania matrix and the formation ofoptically scattering films.

In general, a semiconductor nanocrystal layer includes a plurality ofsemiconductor nanocrystals distributed in a metal oxide matrix. Thesemiconductor nanocrystal layer can be used to amplify optical radiationor produce optical radiation by lasing. In particular, the semiconductornanocrystal layer includes concentrated solids of semiconductornanocrystals, such as close-packed films of semiconductor nanocrystals,that provide high gain to produce optical amplification or lasing overshort amplifier or cavity lengths.

A laser includes an semiconductor nanocrystal layer and a gratingarranged relative to the semiconductor nanocrystal film to providefeedback. A laser can include a layer of a composition, the compositionincluding a grating having a periodicity from 200 nm to 500 nm, from 250nm to 450 nm, or from 300 nm to 400 nm, and a semiconductor nanocrystallayer disposed on the grating. A laser can include a layer of acomposition, the composition including grating and semiconductornanocrystal film on the grating, the film including a plurality ofsemiconductor nanocrystals distributed in a metal oxide matrix. Thesemiconductor nanocrystal layer can include a plurality of semiconductornanocrystals distributed in a metal oxide matrix.

A method of amplifying an optical signal includes directing an opticalbeam into a composition including a grating and a semiconductor filmhaving a plurality of semiconductor nanocrystals distribute in a metaloxide matrix.

A method of forming a laser includes arranging a cavity relative to alayer to provide feedback to the layer. The optical gain medium includesa plurality of semiconductor nanocrystals distributed in a metal oxidematrix.

The layer can be substantially free of defects, reducing losses, such asscatter. The layer can provide gain to an optical signal at an energyequal to or less than the maximum band gap emission of the nanocrystals.The layer also can be capable of providing gain at energies in which aconcentrated solid is substantially free of absorption.

The layer can include greater than 0.2%, greater than 5%, greater than10%, or greater than 15% by volume semiconductor nanocrystals. The eachof the plurality of semiconductor nanocrystals can include a same ordifferent first semiconductor material. The first semiconductor materialcan be a Group II-VI compound, a Group II-V compound, a Group III-VIcompound, a Group III-V compound, a Group IV-VI compound, a GroupI-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-Vcompound, such as, for example, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS,HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP,InAs, InSb, TIN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixtures thereof.Each first semiconductor material can be overcoated with a secondsemiconductor material, such as ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe,CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb,GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, TlSb,PbS, PbSe, PbTe, or mixtures thereof. Each first semiconductor film hasa first band gap and each second semiconductor material has a secondband gap that is larger than the first band gap. Each nanocrystal canhave a diameter of less than about 10 nanometers. The plurality ofnanocrystals can have a monodisperse distribution of sizes. Theplurality of nanocrystals have a plurality of monodisperse distributionof sizes. The plurality of monodisperse distribution of sizes canprovide gain over a broad range of energies or over a plurality ofnarrow ranges, e.g., a FWHM of gain less than 75 nm, in which the gainmaxima occur at a separate energy such that at least some of the narrowranges do not overlap in energy. The concentrated solid of nanocrystalsis disposed on a substrate such as glass. The concentrated solid ofnanocrystals has a thickness greater than about 0.2 microns.

The metal oxide matrix can include a titanium oxide, an aluminum oxide,a silicon oxide, a magnesium oxide, a boron oxide, a phosphorus oxide, agermanium oxide, an indium oxide, a tin oxide, a zirconium oxide, ormixtures thereof.

Stabilization of nanocrystals within a titania matrix in the compositeat volume fractions high enough to observe amplified stimulated emission(ASE) can lead to advantages, such as the observation of ASE at roomtemperature to the creation of more complicated structures showing ASEat multiple wavelengths. Coupling such structures to suitable feedbackwill allow for the development of room temperature lasers that aretunable over a wide spectral window. These matrices may also be usefulfor other non-linear optical applications of nanocrystals, where highnanocrystal density and matrix stability are important.

Other features, objects, and advantages of the invention will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a structure of a nanocrystal and agrating.

FIG. 2 shows the modal gain profile for a CdSe nanocrystal/titaniawaveguide.

FIG. 3 is a photograph of a structure including a nanocrystal and agrating.

FIG. 4 is a plot of the photoluminescence intensity of a CdSenanocrystal/titania film deposited on a DFB grating as a function ofexcitation pump pulse intensity.

FIG. 5 is a plot of the photoluminescence emission spectra of CdSenanocrystals with different radii.

FIG. 6 is a room temperature laser spectrum of a CdSe(ZnS) core-shellnanocrystal/titania DFB device.

DETAILED DESCRIPTION

Fabrication of a surface-emitting distributed feedback grating (DFB)laser structure allows for creation of color-selective DFB lasers. Theunique optical properties of the DFB lasers can be exploited to emit inthe range of wavelengths for 560 nm to 625 nm by varying the size of thenanocrystals stabilized within the nanocrystal/titania matrix. Areduction of the emission line width (FWHM) from ˜30 nm to ˜1 nm is seenin all cases. The observed lasing spectra correspond well with themeasured gain spectra of all these films and is always spectrallyred-shifted with respect to the linear PL emission profile. Under ourdetection conditions, the laser line widths for all the nanocrystallaser devices (FWHM approx. 1-1.5 nm) show a time-integrated envelope ofmany lateral competing DFB lasing modes. Finally, instead of varying theunderlying grating periodicity, the effective refractive index of thenanocrystal/titania matrix can be varied by modulating the volumefraction of the constituent nanocrystals. The ability to independentlyvary both the refractive index as well as the spectral gain position ofsuch systems can allow us to create more complicated nanocrystal lasersoperating simultaneously at multiple wavelengths.

Amplifiers and lasers include gain media for amplifying radiation orproducing radiation by lasing. The gain medium can include a pluralityof semiconductor nanocrystals. The nanocrystals can be illuminated witha light source at an absorption wavelength to cause an emission at anemission wavelength. The emission has a frequency that corresponds tothe band gap of the quantum confined semiconductor material. The bandgap is a function of the size of the nanocrystal. Nanocrystals havingsmall diameters can have properties intermediate between molecular andbulk forms of matter. For example, nanocrystals based on semiconductormaterials having small diameters can exhibit quantum confinement of boththe electron and hole in all three dimensions, which leads to anincrease in the effective band gap of the material with decreasingcrystallite size. Consequently, both the optical absorption and emissionof nanocrystals shift to the blue (i.e., to higher energies) as the sizeof the crystallites decreases.

The emission from the nanocrystal can be a narrow Gaussian emission bandthat can be tuned through the complete wavelength range of theultraviolet, visible, or infrared regions of the spectrum by varying thesize of the nanocrystal, the composition of the nanocrystal, or both.For example, CdSe can be tuned in the visible region and InAs can betuned in the infrared region. The narrow size distribution of apopulation of nanocrystals can result in emission of light in a narrowspectral range. The population can be monodisperse and can exhibit lessthan a 15% rms deviation in diameter of the nanocrystals, preferablyless than 10%, more preferably less than 5%. Spectral emissions in anarrow range of no greater than about 75 nm, preferably 60 nm, morepreferably 40 nm, and most preferably 30 nm full width at half max(FWHM) can be observed. The breadth of the emission decreases as thedispersity of nanocrystal diameters decreases. Semiconductornanocrystals can have high emission quantum efficiencies such as greaterthan 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80%.

The semiconductor forming the nanocrystals can include Group II-VIcompounds, Group II-V compounds, Group III-VI compounds, Group III-Vcompounds, Group IV-VI compounds, Group I-III-VI compounds, GroupII-IV-VI compounds, and Group II-IV-V compounds, for example, ZnS, ZnSe,ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe,PbTe, or mixtures thereof.

Methods of preparing monodisperse semiconductor nanocrystals includepyrolysis of organometallic reagents, such as dimethyl cadmium, injectedinto a hot, coordinating solvent. This permits discrete nucleation andresults in the controlled growth of macroscopic quantities ofnanocrystals. Preparation and manipulation of nanocrystals aredescribed, for example, in U.S. Pat. No. 6,322,901, incorporated hereinby reference in its entirety. The method of manufacturing a nanocrystalis a colloidal growth process. Colloidal growth occurs by rapidlyinjecting an M donor and an X donor into a hot coordinating solvent. Theinjection produces a nucleus that can be grown in a controlled manner toform a nanocrystal. The reaction mixture can be gently heated to growand anneal the nanocrystal. Both the average size and the sizedistribution of the nanocrystals in a sample are dependent on the growthtemperature. The growth temperature necessary to maintain steady growthincreases with increasing average crystal size. The nanocrystal is amember of a population of nanocrystals. As a result of the discretenucleation and controlled growth, the population of nanocrystalsobtained has a narrow, monodisperse distribution of diameters. Themonodisperse distribution of diameters can also be referred to as asize. The process of controlled growth and annealing of the nanocrystalsin the coordinating solvent that follows nucleation can also result inuniform surface derivatization and regular core structures. As the sizedistribution sharpens, the temperature can be raised to maintain steadygrowth. By adding more M donor or X donor, the growth period can beshortened.

The M donor can be an inorganic compound, an organometallic compound, orelemental metal. M is cadmium, zinc, magnesium, mercury, aluminum,gallium, indium or thallium. The X donor is a compound capable ofreacting with the M donor to form a material with the general formulaMX. Typically, the X donor is a chalcogenide donor or a pnictide donor,such as a phosphine chalcogenide, a bis(silyl)chalcogenide, dioxygen, anammonium salt, or a tris(silyl)pnictide. Suitable X donors includedioxygen, bis(trimethylsilyl)selenide ((TMS)₂Se), trialkyl phosphineselenides such as (tri-n-octylphosphine) selenide (TOPSe) or(tri-n-butylphosphine) selenide (TBPSe), trialkyl phosphine telluridessuch as (tri-n-octylphosphine) telluride (TOPTe) orhexapropylphosphorustriamide telluride (HPPTTe),bis(trimethylsilyl)telluride ((TMS)₂Te), bis(trimethylsilyl)sulfide((TMS)₂S), a trialkyl phosphine sulfide such as(tri-n-octylphosphine)sulfide (TOPS), an ammonium salt such as anammonium halide (e.g., NH₄Cl), tris(trimethylsilyl)phosphide ((TMS)₃P),tris(trimethylsilyl)arsenide ((TMS)₃As), ortris(trimethylsilyl)antimonide ((TMS)₃Sb). In certain embodiments, the Mdonor and the X donor can be moieties within the same molecule.

A coordinating solvent can help control the growth of the nanocrystal.The coordinating solvent is a compound having a donor lone pair that,for example, has a lone electron pair available to coordinate to asurface of the growing nanocrystal. Solvent coordination can stabilizethe growing nanocrystal. Typical coordinating solvents include alkylphosphines, alkyl phosphine oxides, alkyl phosphonic acids, or alkylphosphinic acids, however, other coordinating solvents, such aspyridines, furans, and amines may also be suitable for the nanocrystalproduction. Examples of suitable coordinating solvents include pyridine,tri-n-octyl phosphine (TOP), tri-n-octyl phosphine oxide (TOPO) andtris-hydroxylpropylphosphine (tHPP). Technical grade TOPO can be used.

Size distribution during the growth stage of the reaction can beestimated by monitoring the absorption line widths of the particles.Modification of the reaction temperature in response to changes in theabsorption spectrum of the particles allows the maintenance of a sharpparticle size distribution during growth. Reactants can be added to thenucleation solution during crystal growth to grow larger crystals. Bystopping growth at a particular nanocrystal average diameter andchoosing the proper composition of the semiconducting material, theemission spectra of the nanocrystals can be tuned continuously over thewavelength range of 300 nm to 5 microns, or from 400 nm to 800 nm forCdSe and CdTe. The nanocrystal has a diameter of less than 150 Å. Apopulation of nanocrystals has average diameters in the range of 15 Å to125 Å.

The nanocrystal can be a member of a population of nanocrystals having anarrow size distribution. The nanocrystal can be a sphere, rod, disk, orother shape. The nanocrystal can include a core of a semiconductormaterial. The nanocrystal can include a core having the formula MX,where M is cadmium, zinc, magnesium, mercury, aluminum, gallium, indium,thallium, or mixtures thereof, and X is oxygen, sulfur, selenium,tellurium, nitrogen, phosphorus, arsenic, antimony, or mixtures thereof.

The core can have an overcoating on a surface of the core. Theovercoating can be a semiconductor material having a compositiondifferent from the composition of the core. The overcoat of asemiconductor material on a surface of the nanocrystal can include aGroup II-VI compounds, Group II-V compounds, Group III-VI compounds,Group III-V compounds, Group IV-VI compounds, Group I-III-VI compounds,Group II-IV-VI compounds, and Group II-IV-V compounds, for example, ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS,PbSe, PbTe, or mixtures thereof. For example, ZnS, ZnSe or CdSovercoatings can be grown on CdSe or CdTe nanocrystals. An overcoatingprocess is described, for example, in U.S. Pat. No. 6,322,901,incorporated herein by reference in its entirety. By adjusting thetemperature of the reaction mixture during overcoating and monitoringthe absorption spectrum of the core, over coated materials having highemission quantum efficiencies and narrow size distributions can beobtained.

The particle size distribution can be further refined by size selectiveprecipitation with a poor solvent for the nanocrystals, such asmethanol/butanol as described in U.S. Pat. No. 6,322,901, incorporatedherein by reference in its entirety. For example, nanocrystals can bedispersed in a solution of 10% butanol in hexane. Methanol can be addeddropwise to this stirring solution until opalescence persists.Separation of supernatant and flocculate by centrifugation produces aprecipitate enriched with the largest crystallites in the sample. Thisprocedure can be repeated until no further sharpening of the opticalabsorption spectrum is noted. Size-selective precipitation can becarried out in a variety of solvent/nonsolvent pairs, includingpyridine/hexane and chloroform/methanol. The size-selected nanocrystalpopulation can have no more than a 15% rms deviation from mean diameter,preferably 10% rms deviation or less, and more preferably 5% rmsdeviation or less.

The outer surface of the nanocrystal can include a layer of compoundsderived from the coordinating solvent used during the growth process.The surface can be modified by repeated exposure to an excess of acompeting coordinating group to form an overlayer. For example, adispersion of the capped nanocrystal can be treated with a coordinatingorganic compound, such as pyridine, to produce crystallites whichdisperse readily in pyridine, methanol, and aromatics but no longerdisperse in aliphatic solvents. Such a surface exchange process can becarried out with any compound capable of coordinating to or bonding withthe outer surface of the nanocrystal, including, for example,phosphines, thiols, amines and phosphates. The nanocrystal can beexposed to short chain polymers which exhibit an affinity for thesurface and which terminate in a moiety having an affinity for asuspension or dispersion medium. Such affinity improves the stability ofthe suspension and discourages flocculation of the nanocrystal.

The composite can be substantially free of defects such that the filmsprovide gain to optical radiation when excited by a source. Nanocrystalsolids containing defects, i.e., those films not substantially free ofdefects, generate losses, e.g., scatter, such that the films do notgenerate amplified stimulated emission when excited with a source. Thethickness of the film can be, generally, between about 0.2 microns to 10microns, or about 0.25 microns to 6 microns.

Composite of nanocrystals can be formed by redispersing the powdersemiconductor nanocrystals described above in a solvent system and dropcasting films of the nanocrystals from the dispersion. The solventsystem for drop casting depends on the chemical character of the outersurface of the nanocrystal, i.e., whether or not the nanocrystal isreadily dispersible in the solvent system. For example, atetrahydrofuran/ethanol solvent system can be used to drop cast films ofnanocrystals having a surface modified with tris-hydroxylpropylphospine(tHPP) ligand. Nanocrystals having a pyridine surface can be drop castfrom a 9:1 methanol/pyridine solvent system. The drop cast films aredried in an inert atmosphere for about 12 to 24 hours before being driedunder vacuum. Typically, the films are formed on substrates. Thesubstrate can be made from any material that does not react with thenanocrystals. The substrate can be selected such that it is opaque ortransparent at specific energies of optical radiation. The substrate canbe formed in a variety of shapes. Examples of substrate materialsinclude sapphire and silicon. Prior to receiving the film, a substratecan be cleaned by oxygen plasma to remove surface organic contaminants.Alternatively, the hydrophilicity of the substrate's surface can beincreased by first treating the substrate with an etch solution of 7:3ratio of concentrated sulfuric acid and hydrogen peroxide followed bytreatment with 1:1:7 solution of concentrated ammonium hydroxide,hydrogen peroxide and water, to increase the

Transmission electron microscopy (TEM) can provide information about thesize, shape, and distribution of the nanocrystal population. Powderx-ray diffraction (XRD) patterns can provided the most completeinformation regarding the type and quality of the crystal structure ofthe nanocrystals. Estimates of size are also possible since particlediameter is inversely related, via the X-ray coherence length, to thepeak width. For example, the diameter of the nanocrystal can be measureddirectly by transmission electron microscopy or estimated from x-raydiffraction data using, for example, the Scherrer equation. It also canbe estimated from the UV/Vis absorption spectrum. Solid nanocrystalthicknesses can be determined using an ultraviolet/visible spectrometerby measuring the optical absorption of the nanocrystal solid andapplying Beer's law.

Pump-probe laser experiments, such as transient absorption femtosecondlaser experiments, can be used to determine the optical gain ofconcentrated solids of semiconductor nanocrystals. Concentrated solidsof semiconducting nanocrystals, such as close-packed solids, can exhibitgain of optical radiation of about 10 cm⁻¹, 25 cm⁻¹, 50 cm⁻¹, 100 cm⁻¹,or 1,000 cm⁻¹. The onset of gain in films of semiconductor nanocrystalsoccurs when a source excites the nanocrystals to produce electron-hole(e-h) pairs in the semiconductor nanocrystal. Gain can be observed inconcentrated solids of semiconductor nanocrystals at a range oftemperatures (between about 6K to 310K, or above) when the excitationsource produces more than about 1.0, 1.5, or 2.0 e-h pairs persemiconductor nanocrystal. Increasing the source power density toincrease the number e-h pairs can increase the gain of the film.Although described as optical, the excitation source can electrical. Ingeneral, the excitation source should be capable of generating apopulation inversion of the nanocrystal solid.

Gain in the concentrated solids occurs at energies equal to or lowerthan the band gap photoluminescence, i.e., emission. For example, themaximum gain can occur at an energy at or below the maximum band gapemission. The energy of the band gap emission, as described above,depends on the semiconductor material and the size of thequantum-confined nanocrystallite. The energy difference between themaximum of the gain and the emission maximum decreases with decreasingsize of the nanocrystal.

The composite of semiconductor nanocrystals can include nanocrystallitesof the same size and the same semiconductor materials to produce gain ina narrow band of radiation energies, such as in a band of energieshaving a FWHM less than about 75 nm. Alternatively, the semiconductorfilms can be made of different materials, the same material but withdifferent sizes, or both, to produce gain in a broad band of radiationenergies or in multiple narrow bands centered at different radiationenergies.

Referring to FIG. 1, a surface-emitting distributed feedback gratinglaser 10 includes a substrate 20, a gain medium 30 and grating 43. Gainmedium 30 includes a composite of nanocrystals 32 in a metal oxidematrix 33. In operation, a user of amplifier 10 directs an input opticalradiation beam 40 through gain medium 30 and provides an externaloptical radiation beam 50 to excite the gain medium to create apopulation inversion. Provided that the energy of input optical beam 40overlaps the energies at which gain medium 30 facilitates gain,amplifier 10 amplifies optical beam 40 to produce an amplified outputbeam 60.

The general methodology for preparing the structures is based on thefollowing example. First, both CdSe and CdSe(ZnS) core-shellnanocrystals are synthesized using procedures previously described (see,C. B. Murray et al., J. Amer. Chem. Soc. 1993, 115, 8706, C. B. Murray,et al., J. Am. Chem. Soc. 1993, 115, 8706, B. O. Dabbousi et al., J.Phys. Chem. B 1997, 101, 9463). The nativetrioctylphosphine/trioctylphosphine oxide (TOP/TOPO) ligands surroundingthe as-synthesized nanocrystals are replaced with atris-hydroxylpropylphospine (tHPP) ligand, which makes the nanocrystalsstable in alcoholic solvents and provides a tether with which thenanocrystals can be incorporated into a titania matrix. Repeatedcentrifuging/redispersion steps remove the excess TOPO cap. Purifiednanocrystals are then transferred into an inert atmosphere glove box,where they are dispersed in a 1:5 (w/w) solution of tetrahydrofuran andethanol. To the resulting slurry an equal mass of the new ligand, tHPP,is added. Under constant stirring, a stoichiometric equivalent volume(to tHPP) of titanium (IV) butoxide (tBOT) is added. The resultingsolution is allowed to prepolymerize at 60° C. for two hours in theglove box, at the completion of which a clear homogenous solutionresults. Spin coating of this solution on pre-cleaned silica substratesunder controlled humidity conditions (˜20%), and subsequent annealing at200° C. on a hotplate for two minutes, yields a clear,nanocrystal/titania composite film. The relative ratio ofnanocrystal/tHPP to tBOT determines the refractive index of the film,which is sensitively dependent on the volume fraction of nanocrystals.The thickness of the film is coarsely controlled by the ratio oftHPP-tBOT and THF/ethanol and finely controlled by the spin rate of thespin coater. In this sense, nanocrystals are used both as a passivecomponent of the waveguide to tune its refractive index and as theactive lasing medium.

Using this procedure, films with a thickness tunable between 0.2 and 0.7micrometers, with refractive indices between 1.65 and 1.82, andnanocrystal volume fractions as high as 12%, are reproducibly obtained.Film thickness and refractive index were determined by using acombination of profilometry, transmission spectroscopy and ellipsometry.This chemical approach is highly reproducible and easily transferable toa variety of substrates and more complex geometries.

Optical investigations of the films were as follows: The films weremounted in a cryostat and cooled to 80K, or simply kept at roomtemperature. The excitation laser (400 nm, 1 kHz, ˜100 fs) was focusedperpendicular to the substrate near an edge using a cylindrical lens.The width of the stripe on the substrate was 50-100 μm. Thephotoluminescence (PL) intensity from the edge of the substrate wasmeasured as a function of the length of the excitation stripe underconstant photon flux density. A narrow ASE peak was observed emerging asa function of stripe length, with a well-defined threshold, for all thesamples investigated in the spectral range from 560-630 nm. Theline-narrowed emission was observed at the low energy side of thespontaneous PL peak for all samples.

FIG. 2 shows narrow modal gain profiles for nanocrystal/titaniawaveguides tunable with the nanocrystal size covering an energy rangefrom 1.94-2.23 eV. In FIG. 2, normalized nanocrystal/titania waveguidefluorescence is depicted. A gain-induced narrowing of the spontaneousemission FWHM linewidth (from 28 nm to 7 nm) is observed abovethreshold. In all samples these narrow gain profiles have peak valuesthat range from ˜18 cm⁻¹ to ˜50 cm⁻¹ and a FWHM of ˜9-11 nm, usingnanocrystals with a size-distribution <8%. This demonstrates narrow gainprofiles using colloidal semiconductor nanocrystals highlighting thetheoretically predicted quantum confinement effect on these materials.The width of the modal gain profile is predominantly a function of thesize-distribution of the nanocrystals being used.

In general, a surface-emitting DFB laser can be prepared using thefollowing approach. The ability to tune the refractive index of thenanocrystal/titania films then becomes critical in matching the narrowgain profiles (9 nm) to a feedback structure. A second order DFB gratingwas used, where optical feedback is provided through second-order Braggscattering within the thin film, and output coupling of the generatedlaser light is mediated through first-order diffraction normal to thesubstrate. The gratings used were patterned using interferencelithography. See M. L. Schattenburg, et al., J. Vac. Sci. Technol. B,1995, 13, 3007, and M. Farhoud, et al., J. Vac. Sci. Technol. B 1999,17, 3182, each of which is incorporated in its entirety. The use ofinterference lithography provides structures with excellentspatial-phase coherence over large areas, and also allows for theperiodicity of the DFB structure to be easily varied. The gratingperiodicities can be in the range of 200 nm to 500 nm. Gratingperiodicities ranging from 310 nm to 350 nm were fabricated. Thesubstrates used were standard 10-cm diameter silicon wafers, with 1micron of thermal SiO₂ on their surface. The grating pattern wastransferred from a photoresist into the oxide layer using a series ofreactive-ion-etch (RIE) steps. The grating pattern is defined by theBragg condition which is 2Λ=M(λ₀/n_(eff)), where M is the order ofdiffraction, Λ is the periodicity of the grating, λ₀ is the free-spacewavelength of the diffracted light, and n_(eff) is the effectiverefractive index of the active layer. After RIE, the photoresist wasremoved leaving gratings with a rectangular profile 50-nm deep in theSiO₂ layer.

FIG. 3 shows a Scanning Electron Microscope (SEM) micrograph of thinfilms after processing on top of a DFB grating (Λ=330 nm) at 500 nmmagnification. Uniform thickness is evident, though some ripples(probably caused by rapid solvent evaporation) are also seen. Morespecifically, FIG. 3 is a room temperature laser spectrum of a CdSe(ZnS)core-shell (2.5 nm radius CdSe core, ˜2 monolayer ZnS shell)nanocrystal/titania DFB device with a period Λ=350 nm which shows anarrow peak at 626.7 nm with a FWHM=0.8 nm. FIG. 3 illustrateslong-range uniformity of the nanocrystal/titania film. Absence of cracksand pinhole defects through the film are apparent on a sub-20 micrometerrange; characteristics that are a marked improvement over theclose-packed films. The uniformity of the thickness and surfaceroughness of these films is ˜10 nm (rms) and is consistent with atomicforce microscopy investigations on these films. Such characteristicsreduce scattering losses within films and enable us to observe lasingaction at room temperature.

Fabrication of a surface-emitting DFB laser structure then proceeded byspin-coating a nanocrystal/titania thin film on top of the grating.Changing the spinning speed controlled the thickness of the film. Thevolume fraction of nanocrystals, and thus the refractive index of thefilm, was adjusted to match the Bragg condition of the grating with thegain peak of the nanocrystal/titania film. The samples were mounted onthe cold finger of a liquid nitrogen cryostat and cooled to 80 K. The400 nm pulsed excitation light was focused with a cylindrical lens onthe front face of the structure. Photoluminescence emission from thefront of the structure was coupled into a spectrometer and spectrallyanalyzed with a CCD camera.

The photoluminescence emission spectra were characterized. FIG. 4 showsthe photoluminescence emission peak as a function of excitation. Theinsets in FIG. 4 display the spectrally dispersed, time-integratedemission at different pump powers. More specifically, FIG. 4 is a plotof the photoluminescence intensity (λ=607 nm) of a CdSenanocrystal/titania film deposited on a DFB grating (Λ=350 nm) as afunction of excitation pump pulse intensity, shows a clear lasingthreshold. Inset A of FIG. 4 shows the spontaneous emission of thenanocrystal/titania film modulated by the DFB stop band. Inset B of FIG.4 displays the spectrum of nanocrystal laser emission slightly abovethreshold. Inset C of FIG. 4 depicts the spectrum of nanocrystal laseremission further above threshold on a semi-log plot, overlaid with thespectrum from inset B of FIG. 4 for comparison.

In the spontaneous emission regime (inset A of FIG. 4), nanocrystal PLis convoluted with a stop band associated with the one-dimensionalmodulation of the refractive index of the dielectric material. Thisrefractive index modulation results from the thickness modulation of thenanocrystal/titania film caused by the underlying grating. The stop-bandgap is on the order of 10 nm and clearly shows the challenge of matchingthe narrow gain profile of these nanocrystal/titania structures with thegrating parameters. Above the lasing threshold, modes at the edge of thestop band with the highest net gain grow super-linearly as the pumppower is increased (inset B of FIG. 4 and inset C of FIG. 4). Theemission from the front of the structure collapses into a visible laserbeam and the luminescence spot on the film tightens into a bright narrowline.

FIG. 5 demonstrates one advantage of structures including chemicallysynthesized nanocrystals. The size-dependent emission spectrum ofstrongly quantum-confined nanocrystals is exploited to produce DFBlasers that emit in a range of wavelengths from 560 nm to 625 nm byvarying the size of the nanocrystals stabilized within thenanocrystal/titania matrix. FIG. 5 is a plot of the photoluminescenceemission spectra of CdSe nanocrystals with different radii (dashedlines) ((A) 2.7 nm, (B) 2.4 nm, (C) 2.1 nm (D) 1.7 nm (core CdSe)) belowthreshold compared to the spectra above laser threshold (solid lines)for nanocrystal/titania film DFB laser devices at 80 K. The laseremission occurs at A) 621 nm, B) 607 nm, C) 583 nm and D) 560 nm. FIG. 5shows the normalized emission spectra at 80 K, slightly above and belowthreshold, of different nanocrystal/titania matrices when coupled to DFBgratings. A dramatic reduction of the emission line width (FWHM) from˜30 nm to ˜1 nm is seen in all cases. The observed lasing spectracorrespond well with the measured gain spectra of all these films and isalways spectrally red-shifted with respect to the linear PL emissionprofile. All samples are CdSe nanocrystals, except for d) which areCdSe(ZnS) core-shell nanocrystals with ˜2 monolayer of ZnS. Under ourdetection conditions, the laser line widths for all the nanocrystallaser devices (FWHM approx. 1-1.5 nm) show a time-integrated envelope ofmany lateral competing DFB lasing modes.

An additional benefit of using nanocrystal/titania matrices is alsodemonstrated in FIG. 5. The DFB lasers operating at 583, 607 and 625 nmwere prepared using nanocrystals of different sizes, but with the samegrating periodicity (Λ=350 nm) feedback structure. Instead of varyingthe underlying periodicity, the effective refractive index of thenanocrystal/titania matrix was varied by modulating the volume fractionof the constituent nanocrystals. The ability to independently vary boththe refractive index as well as the spectral gain position of suchsystems portend well in developing nanocrystal lasers, operatingsimultaneously at multiple wavelengths.

FIG. 6 demonstrates a room temperature nanocrystal DFB laser. Thetheoretically predicted temperature insensitivity of nanocrystal lasersin the strong confinement regime is observed. FIG. 6 shows theabove-threshold emission profile of a nanocrystal/titania matrix coupledto a DFB grating (Λ=350 nm). A resolution-limited line width (0.8 nm) isobserved from this structure at room temperature; a line width that doesnot vary when cooled down to 80 K.

Laser action has been demonstrated using colloidal CdSe and CdSe(ZnS)core-shell nanocrystals at room temperature as well as at 80 K. Bydeveloping a straightforward chemical route to embed nanocrystals in atitania host matrix, we are able to tune thin film properties toaccurately match their narrow modal gain profiles to a surface-emitting,second-order DFB structure. The precise tunability of these newnanocrystal based lasers is demonstrated by taking advantage of thestrong quantum confinement effect using different sizes and volumefractions of high quality, colloidal CdSe nanocrystals. This nanocrystalthin film system portends well for the incorporation of nanocrystals ofother materials in laser applications, thus opening the possibility ofUV- and IR-operating wet-chemically-prepared nanocrystal lasers. Theease, high throughput, tunability and room temperature operability ofthese nanocrystal systems are featured. The chemical flexibility shouldallow transfer of this technology to a variety of substrates andfeedback schemes.

Other embodiments are within the scope of the following claims.

What is claimed is:
 1. A method of forming a laser comprising: selectinga semiconductor nanocrystal having a diameter and a composition; andplacing the semiconductor nanocrystal on a grating arranged to providefeedback.
 2. The method of claim 1, wherein the diameter of thenanocrystal is selected to emit at a particular wavelength to produce alaser having a particular output energy.
 3. The method of claim 1,wherein the composition of the nanocrystal is selected to emit at aparticular wavelength to produce a laser having a particular outputenergy.
 4. The method of claim 3, wherein the diameter of thenanocrystal is selected to emit at a particular wavelength to produce alaser having a particular output energy.
 5. The method of claim 1,wherein the grating is selected to have a periodicity selected toproduce a laser having a particular output energy.
 6. The method ofclaim 1, further comprising forming a layer including the semiconductornanocrystal on a surface of the grating.
 7. The method of claim 1,further comprising forming a layer including a monodisperse populationof semiconductor nanocrystals on a surface of the grating.
 8. The methodof claim 7, wherein the layer includes greater than 5% by volume ofsemiconductor nanocrystals.
 9. The method of claim 1, wherein thesemiconductor nanocrystal includes a plurality of semiconductornanocrystals, wherein the plurality of semiconductor nanocrystalsinclude a Group II-VI compound, and wherein the plurality ofsemiconductor nanocrystals are distributed in a metal oxide matrix.